Midwest muon experiments carry on East Coast legacy
||The Muon Campus at Fermilab would be the home of the proposed Muon g-2 and Mu2e experiments. symmetry writer Joseph Piergrossi sat down with collaborators from Boston University to learn more about the projects' goals and history. Image: Muon Department/FESS
This spring, scientists at Fermi National Accelerator Laboratory will break ground on the buildings for a Muon Campus. The two initial experiments proposed for the campus draw on three decades of technological advances to turn muons into supersensitive probes for physics beyond the Standard Model.
With the planned Muon g-2 experiment, scientists aim to discover signs of subatomic particles and forces that have eluded detection by other experiments. It will be more sensitive to virtual or hidden particles and forces than any previous experiment of its kind. The Mu2e experiment will test a fundamental symmetry of the quantum world. Scientists have observed the transformation of one type of quark into another, as well as the transition of one type of neutrino into another. The question remains: Can the muon, a charged lepton, change into another type of charged lepton? In particular, can a muon turn into an electron?
Muon g-2 (pronounced g minus two), the first experiment to be installed in the new Muon Campus at Fermilab, has its roots in a muon experiment of the same name that ran from 1997 to 2001 at Brookhaven National Laboratory. The experiment's goal is to measure with high precision the magnetic dipole moment of the muon. The quantity g specifies exactly how much a muon wobbles—or precesses—in a magnetic field.
"The muon is very sensitive to the hidden presence of new physics," says Lee Roberts, professor of physics at Boston University and co-spokesperson for the Muon g-2 experiment.
The Brookhaven Muon g-2 experiment had its inception in 1982, when Yale physicist Vernon Hughes suggested an experiment to measure the magnetic dipole moment of the muon 20 times better than previous experiments run at CERN in the 1970s. He and Roberts were the co-spokespeople for the Brookhaven project and headed the design of the experiment, which eventually involved scientists from 14 institutions in five countries. It required firing muons into a 50-foot-diameter muon storage ring that produced an exceptionally uniform magnetic field. Muons circled the ring many times before decaying. Particle detectors recorded the decays to discern the nature of the muons' precession.
Boston University has had a major stake in the Muon g-2 experiments at Brookhaven and now at Fermilab. In the early 1990s, the university provided the facilities to construct many important components of the muon ring. It was one of a half dozen institutions that "played a crucial role" in the experiment, says Brookhaven's Bill Morse, former resident spokesperson for the Muon g-2 project.
Brookhaven's Muon g-2 experiment produced the best measurement of the wobble, g, to date, and the result doesn't agree with the predictions stemming from the theoretical framework known as the Standard Model of particles and forces. In the absence of quantum effects, g should equal 2. Taking into account all the known particles and forces, theorists can calculate with high precision how much g should differ from 2, which led to the experiment's name. The Brookhaven experiment, however, produced a cliffhanger. It measured g-2 to be outside the Standard Model's predictions with a confidence level of three sigma, meaning that there is a less than 1 in 370 chance that the experiment would observe a deviation of this magnitude from the Standard Model's prediction. For scientists, that is not enough to claim the discovery of a discrepancy, but enough to produce sleepless nights.
After the conclusion of the experiment at Brookhaven, Roberts and other collaborators, including David Hertzog of the University of Washington (the other co-spokesperson of Muon g-2), were looking for a place to make a more precise measurement.
"Hertzog and I explored all around the world, and we concluded the best place was Fermilab," Roberts says. Fermilab's accelerators will enable the new experiment to collect data on more than 20 times the number of muons, an intensity that should lead to a more precise measurement of g-2 and a five-sigma discovery, if the initial observation was not a statistical fluctuation.
The Muon g-2 scientists hope to start recording data at Fermilab in less than three years. Theorists already have some ideas as to why g-2 might not agree with the Standard Model prediction. A theory known as supersymmetry predicts the existence of extra particles and forces, which modify the g-2 calculations.
"Supersymmetry predicts the Muon g-2 results we have so far," says Brookhaven's Morse. "It also predicts that the muon converts into an electron."